Spinal cord injury patients, head injured patients, and stroke victims often benefit from surgical reconstruction using tendon transfers. While numerous transfer procedures exist, performing tendon transfers remains more of an art than a science. We believe that much of the uncertainty regarding the scientific rationale for tendon transfer procedures is based on either a misunderstanding or a complete lack of understanding of the biomechanical properties of muscles used in tendon transfers. Mathematical models have been created to design optimal tendon transfers, however, almost without exception these models are based on the use of generic muscle models whose properties are based on a multitude of scaling and physiological assumptions. We opine that current modeling efforts are almost predestined to failure because the passive biomechanical properties of human muscles are unknown and the normal operating range of almost all human upper extremity muscles has never been measured. In this grant, we present experiments that will define the basic passive biomechanical properties of muscle cells, muscle fiber bundles (cells plus extracellular matrix), and whole muscles in three of the most commonly transferred human muscles-brachioradialis, flexor carpi ulnaris, and pronator teres. These three muscles cover the range of complexity seen in transferred muscles. We will use these data to develop specific guidelines for tendon transfer of these muscles to the VA community. In addition, we will perform technology development of a user-friendly device to measure muscle sarcomere length precisely by reflected laser diffraction (RLD). This method has the advantage over our previous method of being faster and not requiring any tissue dissection. Taken together, these experiments will allow VA surgeons to set the standard in tendon transfer surgery both at the San Diego VA and other VA Medical Centers throughout the country.